This invention relates to a supplemental, vacuum-operated brake system by which the brakes of a vehicle equipped with the supplemental system, in addition to the vehicle""s main brake system, can be actuated from a vacuum system without the application of foot pressure on the brake pedal. In particular, the invention relates to a system in which a vacuum-responsive servo unit mounted in the vehicle and mechanically connected to the brake pedal can be connected pneumatically to the vacuum system of a towing vehicle and controlled either by a valve located in the towing vehicle and in series with the vacuum system in the latter vehicle, or by an electric circuit controlled by a switch actuated by the brake pedal in the towing vehicle.
The term xe2x80x9cvacuumxe2x80x9d is used throughout the following description and claims in the sense in which it is commonly used by persons familiar with automotive brake systems as indicating a pressure much lower than atmospheric pressure although as much as several pounds per square inch higher than absolute vacuum.
When a vehicle being towed either has no brakes or has brakes that are not being operated, the towed vehicle tends to push the towing vehicle forward when the brakes of the latter are applied. This requires that the force applied to the brakes of the towing vehicle be increased, which not only wears out those brakes more rapidly than would otherwise be the case but, in extreme cases of abrupt stops, can make the connected vehicles very unstable.
Trucks that tow large semi-trailers used to haul heavy loads are equipped with air-operated brakes, as are the semi-trailers, so that the force applied to the brake pedal by the driver is greatly augmented by compressed air and applied to both the truck brakes and the trailer brakes in proportion to the driver""s foot pressure on the pedal. Thus, if the driver presses hard on the brake pedal in the truck, all of brakes in both the truck and the semi-trailer will be applied forcefully, while, if the driver applies only light pressure, all of the brakes in the truck and the semi-trailer will be lightly applied. This is a full-time brake system for such vehicles and is not the sort of operation contemplated for this invention. In addition, semi-trailers do not have a separate brake system and are not driven as separate vehicles.
U.S. Pat. No. 5,031,729 to Wittkop et al shows a cement mixer being towed with its front end attached to a crane on a tow truck. The invention includes a bar that has an air cylinder extending longitudinally from one end and a piston rod extending in the same direction from the air cylinder to engage the brake pedal. At the other end of the bar is a yoke to attach that end of the bar to the steering wheel of the vehicle being towed, which prevents the steering wheel of that vehicle from turning and makes it necessary to tow it with its front wheels off the ground. An air compressor in the tow truck supplies compressed air to the cylinder under the control of the brake pedal of the tow truck to force the piston rod to press down on the brake pedal of the cement mixer and apply brake pressure to the rear wheel brakes of the latter to help stop both vehicles.
Another form of brake system is shown in U.S. Pat. No. 4,635,758 to Beard. That patent shows a motor home towing a small car. While the vehicle being towed has all four of its wheels on the ground, the motor home has a second master cylinder to transmit hydraulic pressure from the motor home to the hydraulic system of the car. The brakes of the car are not vacuum-actuated.
U.S. Pat. No. 4,249,643 to Yoder and U.S. Pat. No. 4,763,958 to xc3x96bgard show brake systems involving tractor-trailer vehicles. The trailers never operate independently of the tractors and do not have their own independent brake systems.
The type of vehicle in which the supplemental brake system of this invention is to be used is one that is capable of being driven separately when it is not being towed and which, therefore, has its own main brake system with a brake pedal and brakes. In accordance with this invention, the supplemental brake system is connected to the brake pedal and, thus, to the brakes of the vehicle to apply force to the brake pedal in the absence of any other force applied directly to the brake pedal. In addition, the supplemental brake system of this invention does not interfere with the steering system. Therefore, all of the wheels of the vehicle can be on the ground when the vehicle is being towed.
Such a supplemental brake system is especially, although not exclusively, useful in a vehicle intended to be towed by a motor home or other recreational vehicle. At the end of a journey or a part thereof, the towed vehicle can be easily released so that it can be driven locally while the towing vehicle remains in one spot, either at a temporary location, such as a camp ground, or a more permanent location, such as its home base.
One object of this invention to provide a supplemental brake system separate from the main brake system of a vehicle so that braking can be effected without the necessity of having a driver apply force to the brake pedal of the vehicle.
Another object is to arrange all controls of the supplemental system and all parts connecting the supplemental system to a towing vehicle so that the manipulation and actuation of such parts can be carried out in as unambiguous and fail-safe a manner as possible whether the vehicle that has the supplemental system is connected to or is disconnected from a towing vehicle.
Still another object is to provide a vehicle with a supple-mental brake system that makes it possible to tow that vehicle without supporting the front wheels of that vehicle clear of the pavement.
Yet another object is to make it possible to tow larger vehicles more safely.
A further object is to make both the release of a towed vehicle from a towing vehicle and the reconnection to the towing vehicle easy to accomplish.
Still another object is to provide a two-vehicle system in which actuation of a supplemental brake system in the towed vehicle from the towing vehicle applies the brakes in the towed vehicle to a fixed extent that is less than the maximum amount possible.
After persons skilled in the technology of automotive brake systems have studied the following description, further objects of this invention may become apparent to them.
The supplemental brake system for a vehicle in accordance with this invention includes a vacuum-operated servo unit, such as the type of servo unit used in cruise control systems. The servo unit is rigidly mounted in the vehicle and has an evacuable chamber between a stationary wall and a movable wall, or diaphragm, resiliently pushed away from the stationary wall. Pneumatic means through which a chamber in the servo unit can be evacuated are connected to the chamber, and mechanical means connect the diaphragm to the brake pedal of the vehicle to apply braking force to the pedal in response to vacuum actuation of the servo unit. The supplemental brake system also includes valve means by which the main vacuum line of the primary brake system of the vehicle can be separated from the engine manifold of that vehicle while the supplemental brake system is being used and by which the main vacuum system and the supplemental vacuum line can be sealed from the atmosphere when the vehicle is running free.
A second vehicle to be used to tow a vehicle equipped with a supplemental brake system in accordance with this invention has a vacuum line that extends from a vacuum source, such as the intake manifold in the towing vehicle, to the rear of that vehicle to be connected to the pneumatic means of the towed vehicle to form a complete vacuum path. The towing vehicle has a control operated by the driver of that vehicle to actuate the supplemental brake system of the towed vehicle, but neither the brake system of the towing vehicle nor the primary brake system of the vehicle provided with a supplemental brake system need be modified, and both vehicles can be driven separately.
The braking action obtained by operation of the supplemental brake system is preferably only great enough to reduce the pushing effect of that vehicle on the towing vehicle during a stop but not enough to lock the wheels of the towed vehicle, and it need not be modulated according to the braking force being used in the towing vehicle.
In one embodiment of this invention, a pneumatic switch easily accessible to the driver of the towing vehicle is connected in series with the vacuum line to the rear of that vehicle. When the driver sets the pneumatic switch to a position in which the complete vacuum path is open from the vacuum source in the towing vehicle to the servo unit in the vehicle being towed, the diaphragm wall of the chamber in the servo unit is moved against the pressure of its spring and, through the mechanical connection, applies braking pressure to the brake pedal of the vehicle being towed. In the release position, the diaphragm is allowed to be moved in the opposite direction by a spring in the servo unit, thereby reducing pressure on the brake pedal.
An alternative embodiment makes use of the fact that the chamber of a cruise control servo unit typically has an electrically operated solenoid valve between the chamber and the vacuum line connected thereto. As in the first embodiment, the vacuum source of the towing vehicle is connected by way of the complete vacuum path to the servo unit of the towed vehicle, but unlike the first embodiment, this vacuum connection is always open all the way to an electrically operated valve at a location on the chamber of the servo unit to which the pneumatic connection is attached. Also, as in the first embodiment, an electrical connection extends from the brake light switch of the towing vehicle to the brake lights of the towed vehicle as required by law.
However, in the second embodiment, the brake light line in the towed vehicle is also connected to the electrically operated valve that controls access from the pneumatic connection to the chamber of the servo unit. When the driver of the towing vehicle depresses the brake pedal, this electrically operated valve is actuated by electric current through the brake light switch of the towing vehicle to open the passageway into the chamber from the vacuum path, thereby allowing the chamber to be evacuated to apply a braking force to the brake pedal of the towed vehicle. Since the vacuum path is always evacuated all the way from the vacuum source in the towing vehicle to the servo unit in the towed vehicle, opening the passageway allows the vacuum to have immediate effect in applying pressure to the brake pedal, rather than having to wait until air is evacuated from all of parts of the vacuum path from the vacuum source in the towing vehicle to the chamber in the servo unit. When the driver of the towing vehicle releases pressure on the brake pedal of the towing vehicle, the solenoid is deenergized, which allows air to enter the chamber and return the diaphragm and the brake pedal to their respective non-braking positions. Air may reenter through a small bleed hole in a wall of the chamber.
The invention will be described in greater detail in connection with the drawings.